A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into electric energy. Researchers have been actively studying fuel cells to utilize the fuel cell's potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, is supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical solid oxide fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in contact with a solid impermeable electrolyte. The solid impermeable electrolyte may be a hard ceramic material which allows oxygen ions to flow therethrough. Typically, the reactants, such as hydrogen and oxygen, are fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact and reacted with the electrolyte at the electrolyte interface of the electrodes. Oxygen ions are conducted through the solid impermeable electrolyte from the cathode to the anode where they are reacted with the fuel. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In a solid oxide fuel cell, the reaction at the hydrogen electrode occurs between hydrogen fuel and oxygen ions (O=), which react to form water and release electrons. The reactions at the hydrogen electrode of the solid oxide fuel cell are shown as:H2+O=−>H2O(g)+2e−CO+O=−>CO2+2e−At the oxygen electrode, oxygen and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen into oxygen ions (O=). The reaction at the oxygen electrode of the solid oxide fuel cell is shown as:O2+4e−−>2O=The overall reaction for the solid oxide fuel cell is shown as:H2+½O2−>H2O(g)The flow of electrons from the hydrogen electrode to the oxygen electrode is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
Solid oxide fuel cells require operation at high temperatures to maintain the ionic conductivity of the solid impermeable electrolyte. During operation, solid oxide fuel cells may reach temperatures up to approximately 1,800° F. or 1000° C. One of the main advantages of a solid oxide fuel cell is its ability to utilize hydrocarbon fuels as opposed to requiring a clean supply of hydrogen for operation. Because solid oxide fuel cells operate at high temperature there is the opportunity to reform hydrocarbons within the system either indirectly in a discrete reformer or directly on the anode of the cell. Reducing the operating temperature makes internal reforming more difficult and less efficient, and can mean that more active (and inevitably more expensive) reforming catalysts are required.
Solid oxide fuel cells have high fuel-to-electricity efficiencies, of about 60% normally or 85% with cogeneration. Furthermore, solid oxide fuel cells do not require any infrastructure development as they can be supplied with fuel from existing natural gas supply lines making their operation relatively inexpensive and immediate.
A main disadvantage of solid oxide fuel cells is that the fuel cell requires several hours to reach operating temperatures and begin producing power. This start-up issue is inherent in all high temperature fuel cells. Another issue in solid oxide fuel cells is the slow response to transients. Like other types of conventional fuel cells, the conventional solid oxide fuel cell does not have intrinsic capability to store energy. Intrinsic energy storage could allow for transient response, load leveling, and the ability to accept charge like a battery.